In the past few years there has been a tremendous amount of activity in the area of replacing "subtractive" printed circuit boards with "additive" printed circuit boards. There are major environmental, economic, and marketing reasons for the interest expressed in this technology. Two of the more important reasons are the growth of the electronic industry and the environmental problems associated with traditional copper "subtractive" circuit boards which consume resources, such as the copper foil itself, as well as the process itself which generates hazardous waste.
In an effort to overcome or minimize the disadvantages associated with the production of "subtractive" printed circuit boards, membrane switch circuit boards were developed in the late seventies and early eighties. These circuit boards were generally silver loaded resin inks printed on polyester films. However, they exhibited rather low voltage and current carrying capabilities and were employed principally in the field of simple switches and not as true printed circuit boards.
It is also important to distinguish additive technologies, such as CC-4 boards which rely on electrodeless copper plating to achieve an additive circuit board from the present invention. The former, which indeed is an additive process, nonetheless continues to produce undesirable effluents even though it offers some cost advantages over the more conventional subtractive techniques. On the other hand, the present invention not only avoids the production of undesirable effluents but also provides economic advantages over systems heretofore employed in the production of printed circuit boards.
Two of the more significant advantages secured by the present invention, not achievable heretofore, are (1) a low cost silver based ink providing low resistance values and being U.V.-curable and (2) a low cost silver based solderable ink which also provides low resistance and is U.V.-curable. In both instances the present invention provides a circuit trace whose cross section involves a U.V.-curable material in combination with either silver coated glass or silver coated magnetite spheres.
An inherent problem associated with U.V. technology resides in the fact that the U.V. material itself is non-conductive and represents a significant percentage of the conductive ink composition. In many cases this can be as high as 30 percent U.V.-curable resin and 70 percent conductive material.
While the advantages of the present invention are applicable to planar boards or substrates, it will be appreciated that these same advantages can be secured with non-planar substrates such as, for instance computer keyboards and the like.
The present invention thus relates to systems for securing the above noted advantages and for avoiding the disadvantages associated with known methods of producing printed circuit boards.
One of these systems involves curing the U.V. curable resin component of the U.V. curable ink containing spherical conductive particles by subjecting the same to a U.V. source in a pulsing manner.
Another of these systems involves the use of a magnetic field and while under the influence of the magnetic field curing the U.V. curable resin containing spherical magnetite conductive particles by subjecting the same to a U.V. source whether or not in a pulsing manner.